Monday, 25 November 2013

Placental gas exchange in ground squirrels

Golden-mantled ground squirrel (Callospermophilus lateralis)

(Wikipedia Commons)

Fetal blood tends to have a higher oxygen affinity than maternal blood. This aids oxygen transfer across the placenta. Some mammals (human, sheep) have a special fetal hemoglobin. In many others, hemoglobin is the same in fetus and mother, but in fetal blood it has increased affinity for oxygen because the red blood cells have a low content of DPG (here). This is the case in the few rodents studied so far.

Ground squirrels cannot use this trick. A new study (here) found that even adults have hemoglobin of high oxygen affinity and there is no effect of raising or lowering DPG.

Placenta of the golden-mantled ground squirrel from Carter and Enders (here)

So is oxygen transfer across the placenta of ground squirrels likely to be less efficient than, say, in a rat? Probably not. The same study established that an unusually large Bohr effect (defined here) aids unloading of oxygen to the tissues of ground squirrels. In any placenta there is a double Bohr effect because, as fetal blood takes up oxygen, it unloads carbon dioxide, while the opposite occurs in the maternal blood (discussed in detail here).

The new study concerns ground squirrels living at low and high altitude. The authors speculate that hemoglobin evolved to meet the demands of their fossorial life style. This in turn made it easier to expand into mountainous habitats. They did not consider the possible impact on reproduction - but perhaps they should.